Implantable cardioverter-defibrillators (ICD's) are medical devices that deliver high energy electrical stimulation pulses to cardiac tissue in an attempt to terminate sensed life-threatening cardiac arrhythmias, such as ventricular fibrillation (very fast, chaotic heart rhythms), or slow intrinsic cardiac rates or asystole (a non-beating heart). Hereafter, all such cardiac conditions are referred to generically as a "cardiac arrhythmia." As such, ICD's include sensing circuits for sensing cardiac activity; logic and control circuits for analyzing the sensed cardiac activity to determine if it is representative of a dangerous cardiac arrhythmia, and for controlling the ICD to respond accordingly; and output circuits for generating and delivering high energy stimuli designed to terminate the sensed dangerous cardiac arrhythmia.
One type of ICD, designed for use with an implantable pacemaker, is shown in U.S. Pat. No. 4,989,602 (Sholder et al.), which patent is incorporated herein by reference. The ICD disclosed in the '602 patent advantageously allows the programmable sensing circuits of the pacemaker to be used to help control the ICD. Many other types of ICD's are known and practiced in the art. See, e.g., Cannon, "Implantable Cardioverter-Defibrillator: The Promise and Perils of an Evolving Technology," PACE, Vol. 15, pp. 1-4 (January 1992).
The output circuits of an ICD typically include a charging circuit and one or more output capacitors. The charging circuit is coupled to a low voltage battery, and builds up a high energy charge on the output capacitor over time (0.5-4 seconds). Once the output capacitor has been charged to a prescribed level, which may be from 5 to 40 joules, and once a determination is made by the logic and control circuits of the ICD that a high energy stimulus (commonly referred to as a "defibrillation pulse," a "cardioversion pulse" or simply a "shocking pulse") is needed, the output capacitor is selectively coupled between suitable output terminals of the ICD to appropriate defibrillation leads. The defibrillation leads are essentially insulated electrical conductors that electrically connect the output terminals of the ICD to suitable defibrillation electrodes, judiciously positioned on, in or near a patient's heart, at a distal end of the defibrillation leads. Thus, placing the charge on the output terminals of the ICD effectively places the charge between the defibrillation electrodes, where it discharges as a shocking pulse through the body tissue found between the electrodes.
The effectiveness of the ICD shocking pulse at terminating a given sensed cardiac arrhythmia is determined by numerous factors, such as the energy of the stimulus, the positioning of the electrodes, the design of the electrode, and the type of electrodes. Further, it has been learned in recent years that the effectiveness of a given shocking pulse can be markedly improved by delivering the shocking pulse through a multiplicity of electrodes, each of which is judiciously positioned on, in or near, the cardiac tissue.
When a multiplicity of electrodes are used, it has also been discovered that the polarity of the multiplicity of electrodes relative to each other at the time the discharge is delivered can also influence the effectiveness of the discharge in terminating the sensed cardiac arrhythmia for a given patient. Thus, it is well known to implant a multiplicity of defibrillation leads, e.g., three or more, and position their respective electrodes around and/or in the heart. During the implantation process, such leads are then manually coupled to the positive and negative output terminals of the ICD, and a test discharge is delivered so that its effect can be observed. For example, if three leads are used, two may be connected to the positive output terminal of the ICD, and the other is connected to the negative output terminal. Disadvantageously, it is difficult to predict in advance which of the multiplicity of leads should be coupled to the positive output terminal, and which should be coupled to the negative output terminal. The implanting physician must thus experiment during the implantation process as to what would be the best polarity configuration for a given patient; which experimentation is highly undesirable, particularly when it must be performed by manually connecting different ones of the defibrillation leads together in order to test a given polarity configuration. Hence, there is a need in the art for an ICD that can be quickly and safely implanted in a patient and coupled to a multiplicity of defibrillation leads and electrodes, while still preserving the ability to change the polarity of the electrodes.
Further, it is not uncommon for a patient to exhibit different physiological characteristics at the time of implant than are exhibited after implant. Thus, what might be an optimum polarity configuration for the defibrillation electrodes at the time of implant, may not be optimum a few days, weeks, or even months after implant. Further, as a patient ages, takes certain medications or is exposed to differing environments that create different types of physiological stress, the optimum polarity configuration for the defibrillation leads may likewise change. Thus, the optimum polarity configuration for delivering a shocking pulse for a given patient can very likely change over time. Disadvantageously, existing ICD devices, once implanted, provide no way to alter the polarity of the output electrodes short of explanting the ICD and reversing or switching the leads to achieve appropriate polarity of the lead. Hence, what is needed is an ICD that can be noninvasively programmed as required, in order to selectively alter its output polarity configuration.